Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The photoresist coated on the substrate is next subjected to an image-wise exposure to radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive at lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
The present photoresist compositions are positive-working photoresist, i.e. when they are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the formation of a positive image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many leading edge manufacturing applications today, photoresist resolution on the order of less than 100 nm are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the push toward miniaturization reduces the critical dimensions on the devices.
Photoresists sensitive to short wavelengths, between about 100 nm and about 300 nm are often used where subhalfmicron geometries are required. Particularly preferred are photoresists comprising non-aromatic polymers, a photoacid generator, optionally a dissolution inhibitor, and solvent.
High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. To date, there are three major deep ultraviolet (uv) exposure technologies that have provided significant advancement in miniaturization, and these use lasers that emit radiation at 248 nm, 193 nm and 157 nm. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers, such as those described in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,350,660. On the other hand, photoresists for exposure below 200 nm require non-aromatic polymers since aromatics are opaque at this wavelength. U.S. Pat. No. 5,843,624 and GB 2320718 disclose photoresists useful for 193 nm exposure. Generally, polymers containing alicyclic hydrocarbons are used for photoresists for exposure below 200 nm. Alicyclic hydrocarbons are incorporated into the polymer for many reasons, primarily since they have relatively high carbon hydrogen ratios which improve etch resistance, they also provide transparency at low wavelengths and they have relatively high glass transition temperatures. U.S. Pat. No. 5,843,624 discloses polymers for photoresist that are obtained by free radical polymerization of maleic anhydride and unsaturated cyclic monomers, but the presence of maleic anhydride makes these polymers insufficiently transparent at 157 nm.
Two basic classes of photoresists sensitive at 157 nm, and based on fluorinated polymers with pendant fluoroalcohol groups, are known to be substantially transparent at that wavelength. One class of 157 nm fluoroalcohol photoresists is derived from polymers containing groups such as fluorinated-norbornenes, and are homopolymerized or copolymerized with other transparent monomers such as tetrafluoroethylene (Hoang V. Tran et al Macromolecules 35, 6539, 2002, WO 00/67072, WO 00/17712) using either metal catalyzed or radical polymerization. Generally, these materials give higher absorbencies but have good plasma etch resistance due to their high alicyclic content. More recently, a class of 157 nm fluoroalcohol polymers was described in which the polymer backbone is derived from the cyclopolymerization of an asymmetrical diene such as 1,1,2,3,3-pentafluoro-4-trifluoromethyl-4-hydroxy-1,6-heptadiene (Shun-ichi Kodama et al Advances in Resist Technology and Processing XIX, Proceedings of SPIE Vol. 4690 p 76 2002; WO 02/065212) or copolymerization of a fluorodiene with an olefin (W0 01/98834-A1). These materials give acceptable absorbance at 157 nm, but due to their lower alicyclic content as compared to the fluoro-norbornene polymer, have lower plasma etch resistance. These two classes of polymers can often be blended to provide a balance between the high etch resistance of the first polymer type and the high transparency at 157 nm of the second polymer type.
However, an important limitation to any of these approaches is the availability of a suitable protecting group for fluoroalcohols. In almost all of these approaches, the acid labile protecting groups for the fluoroalcohol moiety has been mainly limited in scope to either acetal type (e.g. MOM (methoxymethyl), or tertiaryalkoxycarbonyl (eg. t-BOC (tert-butyloxycarbonyl) or tertiary alkyl protecting groups. These protecting groups on the perfluoroalcohol moiety are relatively unstable and often undergo partial or complete deprotection during polymerization.
The difficulty in protecting the fluoroalcohol functionality, and the resultant loss of the unexposed photoresist film, has meant that the acid labile functionality can often only be attached to either a methacrylate, acrylate, or norbornenecarboxylic acid repeat unit (which are deleterious to transparency at 157 nm) or to a dissolution inhibitor additive (WO 00/67072, WO 00/17712 Hoang V. Tran et al Macromolecules 35, 6539, 2002). The objective of this invention is to provide a protecting group which would confer good transparency at 157 nm, and would possess high thermal stability towards both synthetic and photoresist processing conditions.
The inventors of this application have found that, surprisingly, aliphatic cyclic polymers can have their fluoroalcohol moieties easily functionalized with an alkyloxycarbonylalkyl(AOCA) group and that this group provides these types of resins with surprising advantages for photoresist applications.
The AOCA group, particularly the tert-butoxycarbonylmethyl (BOCME) group, has had some prior use as a substituent in pharmaceutical applications (WO9533753, DE 2460537). It has also been employed in resist applications where the BOCME group is attached to a phenolic moiety in a resin or as small molecule dissolution inhibitor, and used with radiation at i-line (JP 09211865), 248 nm (JP 20011312065, U.S. Pat. No. 6333436B1, U.S. Pat. No. 6,369,276, JP 08193055) or electron beam and X-ray(JP 2000-376059). In addition, at 248 nm, it has also been employed to protect phenolic resins (JP 08176051). Additionally, the BOCME group has been used to protect some specific monomers for 157 nm photoresists. US 2002/090572 A1 and US2002/0081520 A1 discuss the use of BOCME protected hexafluorobisphenol-A in silsesquioxane based polymers and copolymers, which, however, are very absorbent at 157 nm. Additionally, the use of the BOCME group to protect fluoroalcohol moieties attached to cyclic or alicyclic polymer repeating units are disclosed in WO 02/44811 A2 and EP 1,275,666. In WO 01/74916A1, the abstract discloses a polymer having one segment with an acid decomposable group, one segment derived from a fluoroacrylate and another segment derived from other copolymerizable monomer, and the patent application discloses a large variety of possible polymers. In WO 02/44811 A2, the BOCME group is used to protect a norbornenefluoroalcohol group, however, the polymer is used in a blend with a tetrafluoroethylene copolymer.
Fluorinated analogs of BOCME (e.g. —FC(CH3)CO2t-Bu) have been described as substituents for phenol moieties in applications such as herbicides (EP0044979). US 2002/0061466 discloses the functionalization of the fluorinated analogue of the BOCME group, but on an acrylate monomer.
The present inventors have found that when the AOCA group is used to protect fluoroalcohol moieties it is unexpectedly found to possess higher stability towards thermal cleavage than the t-BOC group or the MOM protecting group. Generally, from a photoresist standpoint, such an increase in thermal stability of the protected fluoroalcohol imparted by use of AOCA, particularly the BOCME group, is desirable as it increases thermal processing latitude and shelf life. Also, from a synthetic standpoint, such a increase in thermal stability is desirable as it helps to increase the yield of protected fluoroalcohol polymers, whether they are made by protection of pre-formed fluoroalcohol polymers or through polymerization of monomers containing a fluoroalcohol bearing polymer protected by AOCA groups. Surprisingly, despite its high thermal stability, the BOCME protecting group, in particular, can be easily removed by photoreleased acid in the exposed resist areas, requiring standard post-exposure bake temperatures to affect cleavage (110° C.-130° C.). It has also been found by the applicants that the functionalization of perfluoroalcohols with AOCA groups results in higher contrast photoresist systems than using tert-butyl carboxylates of photoresist resins containing norbornene-5-carboxylic acid repeat units.
The process of functionalizing the AOCA group to the polymer comprises reacting a polymer containing a hydroxyl group with a reactant that can provide the AOCA group and the reaction proceeds in the presence of a base. Typically, metal bases, especially alkali and alkaline-earth metals, have been used for this reaction, examples of such bases are, sodium hydride, lithium t-butoxide, potassium t-butoxide, etc. U.S. Pat. No. 6,210,859 discloses the reaction of polyhydroxystyrene with t-butyl bromoacetate and morpholino bromoacetate in the presence of 25% tetramethyl ammonium hydroxide aqueous solution.
The inventors of the present application have found that for the synthesis of the aliphatic polymer of the present invention, organic bases, especially ammonium bases, are advantageous over metal bases. Metals from metal bases are harder to remove from the final photoresist than organic bases, and metal bases can leave behind trace amounts of metals which are detrimental to the final product.